Recovery of copper from copper bearing sulphide minerals by bioleaching with controlled oxygen feed

ABSTRACT

A method of recovering copper from a copper bearing sulphide mineral which includes the steps of subjecting the slurry to a bioleaching process, supplying a feed gas which contains in excess of 21% oxygen by volume, to the slurry, and recovering copper from a bioleach residue of the bioleaching process.

BACKGROUND OF THE INVENTION

This invention relates to the recovery of copper from copper bearingsulphide minerals.

Commercial bioleach plants which are currently in operation treatingsulphide minerals, typically operate within the temperature range of 40°C. to 50° C. and rely on sparging air to the bioleach reactors toprovide the required oxygen. Operation at this relatively lowtemperature and the use of air to supply oxygen, limit the rate ofsulphide mineral oxidation that can be achieved. For example carroliteand enargite are relatively slow leaching at temperatures below 50° C.,and treatment at or below this temperature would result in poor andsub-economic metal extraction.

The use of high temperatures between 50° C. and 100° C. greatlyincreases the rate of sulphide mineral leaching.

The solubility of oxygen is however limited at high temperatures and therate of sulphide mineral leaching becomes limited. In the case of usingair for the supply of oxygen, the effect of limited oxygen solubility issuch that the rate of sulphide mineral leaching becomes dependent on andis limited by the rate of oxygen transfer from the gas to the liquidphase.

The bioleaching of secondary copper bearing sulphide minerals issimilarly problematic and to the applicant's knowledge no commercialcopper bioleach plants are in operation.

More particularly chalcopyrite has long been known to be generallyrefractory to bioleaching using mesophiles. A major challenge is theleaching of chalcopyrite, on an industrial scale, using thermophilicmicroorganisms.

SUMMARY OF THE INVENTION

The invention provides a method of recovering copper from a copperbearing sulphide mineral slurry which includes the steps of:

(a) subjecting the slurry to a bioleaching process,

(b) supplying a feed gas which contains in excess of 21% oxygen byvolume, to the slurry, and

(c) recovering copper from a bioleach residue of the bioleachingprocess.

The method may include the step of pre-leaching the slurry prior to thebioleaching process of step (a). The pre-leaching may be effected usingan acidic solution of copper and ferric sulphate.

The method may include the step of removing ferric arsenate from thebioleach residue before step (c). The ferric arsenate may be removed byprecipitation.

The bioleach residue may be subjected to a neutralisation step whichproduces carbon dioxide which is fed to the feed gas of step (b), ordirectly to the slurry.

In step (c) copper may be recovered using a solvent extraction andelectrowinning process. Oxygen which is generated during the copperelectrowinning may be fed to the feed gas of step (b), or directly tothe slurry.

Raffinate, produced by the solvent extraction, may be supplied to atleast one of the following: the bioleaching process of step (a), and anexternal heap leach process.

Oxygen generated during the electrowinning process may be fed to thefeed gas of step (b), or directly to the slurry.

The said slurry may contain at least one of the following: arsenicalcopper sulphides, and copper bearing sulphide minerals which arerefractory to mesophile leaching.

The slurry may contain chalcopyrite concentrates.

As used herein the expression “oxygen enriched gas” is intended toinclude a gas, eg. air, which contains in excess of 21% oxygen byvolume. This is an oxygen content greater than the oxygen content ofair. The expression “pure oxygen” is intended to include a gas whichcontains in excess of 85% oxygen by volume.

Preferably the feed gas which is supplied to the slurry contains inexcess of 85% oxygen by volume ie. is substantially pure oxygen.

The method may include the step of maintaining the dissolved oxygenconcentration in the slurry within a desired range which may bedetermined by the operating conditions and the type of microorganismsused for leaching. The applicant has established that a lower limit forthe dissolved oxygen concentration to sustain microorganism growth andmineral oxidation, is in the range of from 0.2×10⁻³ kg/m³ to 4.0×10⁻³kg/m³. On the other hand if the dissolved oxygen concentration is toohigh then microorganism growth is inhibited. The upper thresholdconcentration also depends on the genus and strain of microorganism usedin the leaching process and typically is in the range of from 4×10⁻³kg/m³ to 10×10⁻³ kg/m³.

Thus, preferably, the dissolved oxygen concentration in the slurry ismaintained in the range of from 0.2×10⁻³ kg/m³ to 10×10−3 kg/m³.

The method may include the steps of determining the dissolved oxygenconcentration in the slurry and, in response thereto, of controlling atleast one of the following: the oxygen content of the feed gas, the rateof supply of the feed gas to the slurry, and the rate of feed of slurryto a reactor.

The dissolved oxygen concentration in the slurry may be determined inany appropriate way, e.g. by one or more of the following: by directmeasurement of the dissolved oxygen concentration in the slurry, bymeasurement of the oxygen content in gas above the slurry, andindirectly by measurement of the oxygen content in off-gas from theslurry, taking into account the rate of oxygen supply, whether in gasenriched or pure form, to the slurry, and other relevant factors.

The method may include the step of controlling the carbon content of theslurry. This may be achieved by one or more of the following: theaddition of carbon dioxide gas to the slurry, and the addition of othercarbonaceous material to the slurry.

The method may extend to the step of controlling the carbon dioxidecontent of the feed gas to the slurry in the range of from 0.5% to 5% byvolume. A suitable figure is of the order of 1% to 1.5% by volume. Thelevel of the carbon dioxide is chosen to maintain high rates ofmicroorganism growth and sulphide mineral oxidation.

The bioleaching process is preferably carried out at an elevatedtemperature. As stated hereinbefore the bioleaching rate increases withan increase in operating temperature. Clearly the microorganisms whichare used for bioleaching are determined by the operating temperature andvice versa. As the addition of oxygen enriched gas or substantially pureoxygen to the slurry has a cost factor it is desirable to operate at atemperature which increases the leaching rate by an amount which morethan compensates for the increase in operating cost. Thus, preferably,the bioleaching is carried out at a temperature in excess of 40° C.

The bioleaching may be carried out at a temperature of up to 100° C. ormore and preferably is carried out at a temperature which lies in arange of from 60° C. to 85° C.

In one form of the invention the method includes the step of bioleachingthe slurry at a temperature of up to 45° C. using mesophilemicroorganisms. These microorganisms may, for example, be selected fromthe following genus groups: Acidithiobacillus (formerly Thiobacillus);Leptosprillum; Ferromicrobium; and Acidiphilium.

In order to operate at this temperature the said microorganisms may, forexample, be selected from the following species: Acidithiobacilluscaldus (Thiobacillus caldus); Acidithiobacillus thiooxidans(Thiobacillus thiooxidans); Acidithiobacillus ferrooxidans (Thiobacillusferrooxidans); Acidithiobacillus acidophilus (Thiobacillus acidophilus);Thiobacillus prosperus; Leptospirillum ferrooxidans; Ferromicrobiumacidophilus; and Acidiphilium cryptum.

If the bioleaching step is carried out at a temperature of from 45° C.to 60° C. then moderate thermophile microorganisms may be used. Thesemay, for example, be selected from the following genus groups:Acidithiobacillus (formerly Thiobacillus); Acidimicrobium;Sulfobacillus; Ferroplasma (Ferriplasma); and Alicyclobacillus.

Suitable moderate thermophile microorganisms may, for example, beselected from the following species: Acidithiobacillus caldus (formerlyThiobacillus caldus); Acidimicrobium ferrooxidans; Sulfobacillusacidophilus; Sulfobacillus disulfidooxidans; Sulfobacillusthermosulfidooxidans; Ferroplasma acidarmanus; Thermoplasma acidophilum;and Alicyclobacillus acidocaldrius.

It is preferred to operate the leaching process at a temperature in therange of from 60° C. to 85° C. using thermophilic microorganisms. Thesemay, for example, be selected from the following genus groups:Acidothermus; Sulfolobus; Metallosphaera; Acidianus; Ferroplasma(Ferriplasma); Thermoplasma; and Picrophilus.

Suitable thermophilic microorganisms may, for example, be selected fromthe following species: Sulfolobus metallicus; Sulfolobus acidocaldarius;Sulfolobus thermosulfidooxidans; Acidianus infernus; Metallosphaerasedula; Ferroplasma acidarmanus, Thermoplasma acidophilum; Thermoplasmavolcanium; and Picrophilus oshimae.

The slurry may be leached in a reactor tank or vessel which is open toatmosphere or substantially closed. In the latter case vents for off-gasmay be provided from the reactor.

According to a different aspect of the invention there is provided amethod of recovering copper from a slurry containing copper bearingsulphide minerals which includes the steps of bioleaching the slurryusing suitable microorganisms at a temperature in excess of 40° C.,controlling the dissolved oxygen concentration in the slurry within apredetermined range, and recovering copper from a bioleach residue.

The bioleaching may be carried out at a temperature in excess of 60° C.

The dissolved oxygen concentration may be controlled by controlling theaddition of gas which contains in excess of 21% oxygen by volume to theslurry.

Preferably the gas contains in excess of 85% by volume.

The bioleach residue may be subjected to a separation step to produceresidue solids and solution and the copper may be recovered from thesolution in any appropriate way, for example by means of a solventextraction and electrowinning process.

The invention also extends to a method of enhancing the oxygen masstransfer coefficient from a gas phase to a liquid phase in a copperbearing sulphide mineral slurry which includes the step of supplying afeed gas containing in excess of 21% oxygen by volume to the slurry.

The feed gas preferably contains in excess of 85% oxygen by volume.

The invention further extends to a method of bioleaching an aqueousslurry containing copper bearing sulphide minerals which includes thesteps of bioleaching the slurry at a temperature above 40° C. andmaintaining the dissolved oxygen concentration in the slurry in therange of from 0.2×10⁻³ kg/m³ to 10×10⁻³ kg/m³.

The dissolved oxygen concentration may be maintained by supplying gascontaining in excess of 21% oxygen by volume to the slurry. Thetemperature is preferably in the range of from 60° C. to 85° C.

The invention further extends to a plant for recovering copper from acopper bearing sulphide mineral slurry which includes a reactor vessel,a source which feeds a copper bearing sulphide mineral slurry to thevessel, an oxygen source, a device which measures the dissolved oxygenconcentration in the slurry in the vessel, a control mechanism whereby,in response to the said measure of dissolved oxygen concentration, thesupply of oxygen from the oxygen source to the slurry is controlled toachieve a dissolved oxygen concentration in the slurry within apredetermined range, and a recovery system which recovers copper from abioleach residue from the reactor vessel.

The oxygen may be supplied in the form of oxygen enriched gas orsubstantially pure oxygen.

The reactor vessel may be operated at a temperature in excess of 60° C.and preferably in the range of 60° C. to 85° C.

The plant may include a pre-leaching stage for leaching the copperbearing sulphide mineral slurry before the slurry is fed to the reactorvessel. In the pre-leaching stage use may be made of an acidic solutionof copper and ferric sulphate.

Various techniques may be used for controlling the supply of oxygen tothe slurry and hence for controlling the dissolved oxygen concentrationin the slurry at a desired value. Use may for example be made of valveswhich are operated manually. For more accurate control use may be madeof an automatic control system. These techniques are known in the artand are not further described herein.

As has been indicated oxygen and carbon dioxide may be added to theslurry in accordance with predetermined criteria. Although the additionof these materials may be based on expected demand and measurement ofother performance parameters, such as iron(II) concentration, it ispreferred to make use of suitable measurement probes to sample theactual values of the critical parameters.

For example use may be made of a dissolved oxygen probe to measure thedissolved oxygen concentration in the slurry directly. To achieve thisthe probe is immersed in the slurry. The dissolved oxygen concentrationmay be measured indirectly by using a probe in the reactor off-gas or bytransmitting a sample of the off-gas, at regular intervals, to an oxygengas analyser. Again it is pointed out that measuring techniques of thistype are known in the art and accordingly any appropriate technique canbe used.

A preferred approach to the control aspect is to utilise one or moreprobes to measure the dissolved oxygen concentration in the slurry,whether directly or indirectly. The probes produce one or more controlsignals which are used to control the operation of a suitable valve orvalves, eg. solenoid valves, automatically so that the supply of oxygento an air stream which is being fed to the slurry is variedautomatically in accordance with real time measurements of the dissolvedoxygen concentration in the slurry.

Although it is preferred to control the addition of oxygen to a gasstream which is fed to the slurry a reverse approach may be adopted inthat the oxygen supply rate to the reactor vessel may be maintainedsubstantially constant and the rate of supply of the sulphide mineralslurry to the reactor vessel may be varied to achieve a desireddissolved oxygen concentration.

The invention is not limited to the actual control technique employedand is intended to extend to variations of the aforegoing approaches andto any equivalent process.

The method of invention is of particular benefit to chalcopyriteconcentrates, which are more-or-less refractory to leaching at mesophileoperating temperatures. The method of the invention therefore opens thedoor to commercial thermophile leaching of chalcopyrite which to theapplicant's knowledge was previously not possible. The added benefits ofa high specific reactor sulphide oxidation duty and reduced specificpower requirement for oxidation, while still advantageous, are of lesssignificance in this instance.

Additionally copper bearing sulphide flotation concentrates frequentlycontain chalcocite and the method of the invention is of particularbenefit, because chalcocite has a high leaching rate, even at typicalmesophile operating temperatures, which is further increased at thehigher temperatures used with moderate and extreme thermophiles. Thusthe benefits of the invention, including a high specific reactorsulphide oxidation duty and reduced specific power requirement foroxidation, will be particularly beneficial during the bioleaching ofcopper bearing sulphide concentrates containing chalcocite, even attypical mesophile operating temperatures.

Copper may be recovered from solution by any appropriate process, forexample solvent extraction followed by electrowinning, ironprecipitation, or by resin-in-pulp applied to the slurry, followed byelectrowinning.

If electrowinning is selected as the production method for copper, theoxygen generated at the anode in the electrowinning process may be usedto supplement that used in the bioleach process, reducing the capitaland operating costs required for oxygen production.

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DESCRIPTION OF PREFERRED EMBODIMENTS

General Principles

The limitation of low oxygen solubility during bioleaching, using air,at high temperatures, which in turn limits the rate of reaction,requires enrichment of the air with oxygen ie. air with an oxygencontent greater than 21% by volume, or the use of pure oxygen (definedas being greater than 85% oxygen by volume). The use of oxygen enrichedair or pure oxygen overcomes the limited rate of reaction due to oxygensupply constraints, but has two major disadvantages:

a) the provision of oxygen enriched air or pure oxygen is expensive andrequires a high utilisation (>60%) of the oxygen to warrant theadditional expense; and

b) if the oxygen level in solution becomes too high microorganism growthis prevented and sulphide mineral bioleaching stops.

Therefore, in order to realise the benefits of high rates of sulphidemineral leaching at high temperatures in commercial bioleaching plants,the drawbacks of requiring expensive oxygen and the risk of failure ifthe dissolved oxygen levels become too high must be overcome.

The bioleaching of sulphide minerals at an elevated temperature resultsin a high rate of sulphide mineral oxidation, but is dependent on thesupply of oxygen and carbon dioxide to maintain high rates of sulphidemineral oxidation and of microorganism growth at adequate rates. Theabsorption of oxygen and carbon dioxide in the bioleaching reactor islimited, in each case, by the rate of mass transfer from the gas phaseinto the solution phase. For oxygen the rate of oxygen absorption isdefined by equation (1) as follows:

R=M.(C*−C _(L))  (1)

where:

R=Oxygen demand as mass (kg) per unit volume (m³) per unit time(s)(kg/m³/s),

M=Oxygen mass transfer coefficient in reciprocal seconds (s⁻¹),

C*=Saturated dissolved oxygen concentration as mass (kg) per unit volume(m³) (kg/m³), and

C_(L)=Dissolved oxygen concentration in solution as mass (kg) per unitvolume (m³) (kg/m³).

The factor (C*−C_(L)) is referred to as the oxygen driving force. Asimilar equation may be used to describe the rate of carbon dioxidesupply to the solution. If the sulphide mineral oxidation rate isincreased the oxygen demand increases proportionately. To meet a higheroxygen demand either the oxygen mass transfer coefficient (M) or theoxygen driving force (C*·C_(L)) must be increased.

An increase in the oxygen mass transfer coefficient may be achieved byincreasing the power input to the bioleach reactor mixer. This improvesgas dispersion in the sulphide mineral slurry. With this approach,however, an increase in the oxygen mass transfer coefficient of, forexample, 40% requires an increase in the power input to the mixer by afactor of as much as 200%, with a commensurate increase in operatingcosts.

The oxygen driving force may be increased by increasing the saturateddissolved oxygen concentration C* and reducing the dissolved oxygencontent or concentration C_(L).

Microorganism population growth is limited or prevented if the dissolvedoxygen concentration C* reaches too high a level. A concentration levelabove 4×10⁻³ kg/m³ has been found to be detrimental to Sulfolobus-likestrains. Certain Acidithiobacillus strains, however, have been found tobe tolerant to dissolved oxygen concentrations of up to 10×10⁻³ kg/m³.

The applicant has established that a lower limit for the dissolvedoxygen concentration to sustain microorganism growth and mineraloxidation is in the range of from 0.2×10⁻³ kg/m³ to 4.0×10⁻³ kg/m³.Thus, in order to provide an adequate, or optimum, supply of oxygen, thedissolved oxygen concentration in the sulphide mineral slurry must bemonitored and, where appropriate, the addition of oxygen to the sulphidemineral slurry must be controlled in order to maintain the minimumdissolved oxygen concentration in solution at a value of from 0.2×10⁻³kg/m³ to 4.0×10⁻³ kg/m³.

On the other hand the dissolved oxygen concentration must not exceed anupper threshold value at which microorganism growth is prevented. It ispointed out that the upper threshold concentration depends on the genusand strain of microorganism used in the bioleaching process. A typicalupper threshold value is in the range of from 4×10⁻³ kg/m³ to 10×10⁻³kg/m³.

As has been previously indicated the rate of sulphide mineral oxidation,which can be achieved when operating at a relatively low temperature ofthe order of from 40° C. to 55° C., is limited. In order to increase therate of oxidation it is desirable to make use of thermophiles and tooperate at temperatures in excess of 60° C. Any suitable microorganismcapable of operating within this temperature range may be used. Theoptimum operating temperature is dependent on the genus and type ofmicroorganism used. Thus moderate thermophiles of the type Sulfobacillusare suitable for operating at a temperature of up to 65° C. Thermophilesof the type Sulfolobus are suitable for operating at temperatures offrom 60° C. to at least 85° C. Sulfolobus metallicus, for example, showsoptimal growth in the temperature range of from 65° C. to 70° C.

The applicant has established that the operation of the bioleachingprocess, using a gas enriched with oxygen, or pure oxygen, as theoxidant, at elevated temperatures of from 40° C. to 85° C.: increasesthe specific sulphide oxidation duty of the reactor considerably;results in an unexpected and significantly enhanced oxygen mass transferrate; increases the oxygen utilisation, providing that the dissolvedoxygen concentration is controlled above the point where microorganismgrowth and mineral oxidation are prevented and below the point at whichmicroorganism growth is inhibited; and the overall power required forthe oxidation of sulphide minerals is significantly reduced.

The method of the invention represents a significant improvementcompared to a bioleach operation carried out at a temperature of from40° C. to 45° C. with air.

The controlled addition of oxygen enriched air or pure oxygen directlyinto the bioreactor improves the oxygen utilisation efficiency. Theoxygen utilisation for a conventional commercial bioleach plant (atleast 100 m³ in volume) operating at from 40° C. to 45° C. with air maybe expected to achieve a maximum oxygen utilisation factor of from 40%to 50%. Consequently only 40% to 50% of the total mass of oxygensupplied to the bioleach plant is used to oxidise the sulphide minerals.With the method of the invention the oxygen utilisation is significantlyhigher, of the order of from 60% to 95%. The higher oxygen utilisationis achieved by controlled oxygen addition and results from the enhancedoxygen mass transfer rate and by operating at low dissolved oxygenconcentrations in the solution phase.

It will be appreciated that although high oxygen demand in bioleachreactors has come about primarily by the use of higher temperatures,rapidly leaching sulphide minerals at temperatures below 60° C., usingmesophile or moderate thermophile microorganisms, will have similarlyhigh oxygen demands. The method of the invention is therefore notrestricted to suit thermophiles or extreme thermophiles, but alsomesophile and moderate thermophile microorganisms.

Another advantage of using air enriched with oxygen or pure oxygen isthat the evaporation losses are reduced, because there is less inert gasremoving water vapour from the top of the reactor. This is particularlyimportant in areas where water is scarce or expensive.

in carrying out the method of the invention the temperature of theslurry in the bioleach vessel or reactor may be controlled in anysuitable way known in the art. In one example the bioleach reactor isinsulated and heating takes place by means of energy which is releasedby the oxidation of sulphides. The temperature of the slurry isregulated using any suitable cooling system, for example an internalcooling system.

Table 1 shows typical data for specific sulphide oxidation duty andoxygen utilisation, when bioleaching with air at 40° C. to 45° C., intwo commercial bioreactors, Plant A and Plant B respectively, (greaterthan 100m³ in volume).

TABLE 1 Commercial Bioreactor Performance Results Description UnitsPlant A Plant B Reactor temperature ° C. 42 40 Reactor operating volumem³ 471 896 Oxygen utilisation % 37.9 43.6 Typical dissolved oxygenconcentration mg/l 2.5 2.7 Oxygen mass transfer coefficient s⁻¹ 0.0470.031 Specific oxygen demand kg/m³/day 21.6 14.8 Specific sulphideoxidation duty kg/m³/day 8.9 5.7 Specific power consumption per kgkWh/kgS²⁻ 1.7 1.8 sulphide oxidised

At low temperatures (40° C.-50° C.), with air as the inlet gas, whichapplies to the results for the commercial reactors, Plant A and Plant B,presented in Table 1, the oxygen utilisations achieved are expected andthe oxygen mass transfer coefficients (M) correspond to the applicant'sdesign value. The applicant has determined that if the method of theinvention were to be applied to Plant A, the plant performance would besignificantly increased, as indicated by the results presented in Table2.

TABLE 2 Predicted Improvement In Commercial Bioreactor Performance PlantA - using Plant A - typical the method of Units operation the inventionReactor temperature ° C. 42 77 Microbial type strain — AcidithiobacillusSulfolobus Inlet gas oxygen % by volume 20.9 90.0 content Oxygenutilisation % 37.9 93.0 Typical dissolved mg/l 2.5 2.5 oxygenconcentration Specific oxygen kg/m³/day 21.6 59.5 demand Specificsulphide kg/m³/day 8.9 24.5 oxidation duty Specific power kWh/kgS²⁻ 1.71.2 consumption per kg sulphide oxidised

The results clearly show the benefit of the invention in achievinghigher rates of reaction by the combination of bioleaching at hightemperature, adding oxygen enriched gas and by controlling the dissolvedoxygen concentration to a predetermined low level (e.g. 0.2×10⁻³ kg/m³to 4.0×10⁻³ kg/m³). The specific sulphide oxidation duty of the reactoris increased by almost threefold. Clearly the upper dissolved oxygenconcentration should not be increased above a value at whichmicroorganism growth is inhibited or stopped.

Even though additional capital for the production of oxygen is required,the savings in reactor and other costs at least offset this additionalexpense. Additionally, the specific power consumption per kg sulphideoxidised is decreased by approximately one-third. In a plant oxidising300 tonnes of sulphide per day, the power saving, assuming a power costof US$0.05 per kWh, would amount to US$2.8 million per annum. The highoxygen utilisation and increased specific sulphide oxidation capacity ofthe reactor represent in combination a considerable improvement overconventional bioleaching practice conducted at lower temperatures, withoxygen supplied by air.

Bioleaching Plant

FIG. 1 of the accompanying drawings shows a bioleaching plant 10 inwhich bioleaching is carried out, in accordance with the principles ofthe invention.

The plant 10 includes a bioreactor 12 with an agitator or mixer 14 whichis driven by means of a motor and gearbox assembly 16.

In use a tank or vessel 18 of the reactor contains a sulphide mineralslurry 20. An impeller 22 of the agitator is immersed in the slurry andis used for mixing the slurry in a manner which is known in the art.

A probe 24 is immersed in the slurry and is used for measuring thedissolved oxygen concentration in the slurry. A second probe 26, insidethe tank 18 above the surface level 28 of the slurry, is used formeasuring the carbon dioxide content in the gas 30 above the slurry 20.

An oxygen source 32, a carbon dioxide source 34 and an air source 36 areconnected through respective control valves 38, 40 and 42 to a spargingsystem 44, positioned in a lower zone inside the tank 18, immersed inthe slurry 20.

The probe 24 is used to monitor the dissolved oxygen concentration inthe sulphide mineral slurry 20 and provides a control signal to acontrol device 46. The control device controls the operation of theoxygen supply valve 38 in a manner which is known in the art but inaccordance with the principles which are described herein in order tomaintain a desired dissolved oxygen concentration in the slurry 20.

The probe 26 measures the carbon dioxide content in the gas above thesulphide mineral slurry 20. The probe 26 provides a control signal to acontrol device 48 which, in turn, controls the operation of the valve 40in order to control the addition of carbon dioxide from the source 34 toa gas stream flowing to the sparger 44.

The air flow rate from the source 36 to the sparger 44 is controlled bymeans of the valve 42. Normally the valve is set to provide a more orless constant flow of air from the source 36 to the sparger and theadditions of oxygen and carbon dioxide to the air stream are controlledby the valves 38 and 40 respectively. Although this is a preferredapproach to adjusting the oxygen and carbon dioxide contents in the airflow to the sparger other techniques can be adopted. For example it ispossible, although with a lower degree of preference, to adjust the airstream flow rate and to mix the adjustable air stream with a steadysupply of oxygen and a variable supply of carbon dioxide, or vice versa.Another possibility is to have two separate air stream flows to whichare added oxygen and carbon dioxide respectively. Irrespective of thetechnique which is adopted the objective remains the same, namely tocontrol the additions of oxygen and carbon dioxide to the slurry 20.

Slurry 50 is fed from a slurry feed source 52 through a control valve 54and through an inlet pipe 56 into the interior of the tank 18. Theslurry feed rate may be maintained substantially constant, byappropriate adjustment of the valve 54, to ensure that slurry issupplied to the tank 18 at a rate which sustains an optimum leachingrate. The supplies of air, oxygen and carbon dioxide are then regulated,taking into account the substantially constant slurry feed rate, toachieve a desired dissolved oxygen concentration in the slurry 20 in thetank, and a desired carbon dioxide content in the gas 30 above theslurry. Although this is a preferred approach it is apparent that theslurry feed rate could be adjusted, in response to a signal from theprobe 24, to achieve a desired dissolved oxygen concentration in theslurry. In other words the rate of oxygen addition to the slurry may bekept substantially constant and the slurry feed rate may be variedaccording to requirement.

Another variation which can be adopted is to move the probe 24 from aposition at which it is immersed in the slurry to a position designated24A at which it is located in the gas 30 above the level 28. The probethen measures the oxygen contained in the gas above the slurry ie. thebioreactor off-gas. The oxygen content in the off-gas can also be usedas a measure to control the dissolved oxygen concentration in theslurry, taking any other relevant factors into account.

Conversely it may be possible to move the carbon dioxide probe 26(provided it is capable of measuring the dissolved carbon dioxidecontent) from a position at which it is directly exposed to the gas 30to a position designated 26A at which it is immersed in the slurry inthe tank. The signal produced by the probe at the position 26A is thenused, via the control device 48, to control the addition of carbondioxide from the source 34 to the air stream from the source 36.

Although the carbon dioxide source 34, which provides carbon dioxide ingas form, is readily controllable and represents a preferred way ofintroducing carbon into the slurry 20, it is possible to add suitablecarbonate materials to the slurry 50 before feeding the slurry to thereactor. Carbonate material may also be added directly to the sulphidemineral slurry 20 in the reactor. In other cases though there may besufficient carbonate in the sulphide mineral slurry so that it is notnecessary to add carbon, in whatever form, to the slurry nor to controlthe carbon content in the slurry.

It is apparent from the aforegoing description which relates to thegeneral principles of the invention that the supply of oxygen to theslurry is monitored and controlled to provide a desired dissolved oxygenconcentration level in the slurry 20. This can be done in a variety ofways eg. by controlling one or more of the following in an appropriatemanner namely: the slurry feed rate, the air flow rate from the source36, the oxygen flow rate from the source 32, and any variation of theaforegoing.

The carbon dioxide flow rate is changed in accordance with the total gasflow rate to the sparger 44 in order to maintain a concentration in thegas phase, i.e. in the gas stream to the reactor, of from 0.5% to 5%carbon dioxide by volume. This carbon dioxide range has been found tomaintain an adequate dissolved carbon dioxide concentration in theslurry, a factor which is important in achieving effective leaching.

The addition of oxygen to the sulphide mineral slurry 20 is controlledin order to maintain the minimum dissolved oxygen concentration insolution at a value of from 0.2×10⁻³ kg/m³ to 4.0×10⁻³ kg/m³. The upperthreshold value depends on the genus and strain of microorganism used inthe bioleaching process and typically is in the range of from 4×10⁻³kg/m³ to 10×10⁻³ kg/m³.

FIG. 1 illustrates the addition of oxygen from a source 32 of pureoxygen. The pure oxygen can be mixed with air from the source 36. Anyother suitable gas can be used in place of the air. The addition ofoxygen to air results to what is referred to in this specification asoxygen enriched gas ie. a gas with an oxygen content in excess of 21% byvolume. It is possible though to add oxygen substantially in pure formdirectly to the slurry. As used herein pure oxygen is intended to mean agas stream which contains more than 85% oxygen by volume.

The temperature in the bioleach reactor or vessel may be controlled inany appropriate way using techniques which are known in the art. In oneexample the lank 18 is insulated and heating takes place by means ofenergy which is released by the oxidation of sulphides. The temperatureof the slurry 20 is regulated using an internal cooling system 70 whichincludes a plurality of heat exchanger cooling coils 72 connected to anexternal heat exchanger 74.

The vessel 18 may be substantially sealed by means of a lid 80. Smallvents 82 are provided to allow for the escape of off-gas. The off-gasmay, if required, be captured or treated in any appropriate way beforebeing released to atmosphere. Alternatively, according to requirement,the tank 18 may be open to atmosphere.

The microorganisms chosen for the leaching process will determine theleaching temperature, and vice versa. The applicant has found that apreferred operating temperature is above 60° C., for example in therange of 60° C. to 85° C. In this range thermophilic microorganisms, inany appropriate combination, are employed. In the range of from 45° C.to 60° C., on the other hand, moderate thermophiles are employed whileat temperatures below 45° C. mesophiles are used. These microorganismsmay, for example, be chosen from those referred to hereinbefore.

Although the benefit of adding oxygen to the slurry which is to beleached, by making use of oxygen enriched air or, more preferably, bymaking use of substantially pure oxygen ie. with an oxygen content inexcess of 85%, is most pronounced at high temperatures at which greaterleaching rates are possible, a benefit is nonetheless to be seen whenoxygen enriched air or substantially pure oxygen is added to the slurryat lower temperatures, of the order of 40° C. or even lower. At thesetemperatures the leaching rates are slower than at elevated temperaturesand although an improvement results from using oxygen enriched air thecost thereof is generally not warranted by the relatively small increasein leaching rate.

Test Results

The importance of maintaining an adequate supply of oxygen and hence asufficiently high dissolved oxygen concentration to sustainmicroorganism growth and mineral oxidation is shown in the resultspresented in FIG. 2. If the dissolved oxygen is allowed to drop below1.5 ppm, and particularly below 1.0 ppm, biooxidation becomes unstable,which is indicated by higher iron(II) concentrations in solution, ofgreater than 2 g/l. At consistent levels of biooxidation, achieved bymaintaining a dissolved oxygen concentration above 1.5 ppm, in thisexperiment, iron(II) is rapidly oxidised to iron(III), and iron(II)concentrations remain generally below 1.0 g/l.

The results presented in FIG. 2 were obtained from operation of a firstor primary reactor of a continuous pilot plant treating a chalcopyriteconcentrate at a feed solids concentration of 10% by mass and atemperature of 77° C., with Sulfolobus-like archaea.

The effect of increasing the oxygen content of the feed gas to abloreactor and controlling the dissolved oxygen concentration, inaccordance with the principles of the invention, was tested in anexperiment using a 5 m³ bioreactor which was operated with a continuouspyrite or blended pyrrhotite and pyrite flotation concentrate feed, at atemperature of about 77° C., using a mixed culture of Sulfolobus-likearchaea and a solids density of 10% by mass. The carbon dioxide contentin the bioleach inlet gas was controlled at a level of between 1 and 1.5% by volume. The dissolved oxygen concentration was generally within therange 0.4×10⁻³ kg/m³ to 3.0×10⁻³ kg/m³. The results of the experimentare presented in FIG. 3.

From the graphs presented in FIG. 3 it is clear that, when sparging withair (enriched with carbon dioxide: 20.7% oxygen and 1.0% carbondioxide), the maximum oxygen demand (directly proportional to thesulphide oxidation duty) was limited to 11.3 kg/m³/day, since thedissolved oxygen concentration which was achievable using air only (i.e.not enriched with oxygen) was just sufficient to maintain microorganismgrowth.

By controlling the oxygen content of the inlet gas, the oxygen additionrate, and the dissolved oxygen concentration in the slurry in the rangeof 0.4×10⁻³ kg/m³ to 3.0×10⁻³ kg/m³, the oxygen demand, i.e. thesulphide mineral oxidation rate, was increased dramatically. Thedissolved oxygen concentration was controlled to a low value, but abovethe minimum limit for successful microorganism growth, so that theutilisation of oxygen was maximised. The results show the oxygen demand,or sulphide oxidation duty, was increased by over threefold. Thus byincreasing the oxygen content in the inlet gas from 20.7% to a maximumof 90.8% the specific oxygen demand was increased from 11.3 kg/m³/day to33.7 kg/m³/day. In addition, by controlling the dissolved oxygenconcentration to a low value, but above the minimum value for sustainedmicroorganism growth, the oxygen utilisation was maximised. The oxygenutilisation showed a general increase with an increase in the oxygencontent of the inlet gas from 29% (for an inlet gas oxygen content of20.7%) to 91% (for inlet gas containing 85.5% oxygen).

The high oxygen utilisations achieved of well over 60% are much betterthan expected. Analysis of the results indicates that the oxygen masstransfer coefficient (M), as defined by equation (1), is significantlyand unexpectedly enhanced for operation of the bioreactor at a hightemperature (77° C.) and with a high oxygen content in the inlet gas(from 29% to 91% in the experiment). In fact, the oxygen mass transfercoefficient (M) is increased by a factor of 2.69, on average, comparedto the applicant's design value. This enhancement is after consideringthe improvement in the mass transfer coefficient due to temperature,which would be expected to increase the value of M by a factor of 1.59for a temperature increase from 42° C. to 77° C., according to thetemperature correction factor. This correction factor has beendemonstrated experimentally to be valid for a temperature in the rangeof from 15° C. to 70° C.

The determination of the enhanced oxygen mass transfer coefficient isshown from the results presented in FIG. 4, where the oxygen demanddivided by the design oxygen mass transfer coefficient (M_(design)) isplotted against the oxygen driving force, as defined in equation (1).The slope of the regression line plotted through the data indicates theenhancement in the oxygen mass transfer coefficient by a factor of 2.69.

Process Examples

The Inventive principles in the preceding section have been described inthe context of sulphide minerals in general and, as will be appreciatedby those skilled in the art, can be applied to copper bearing sulphideminerals in particular.

FIG. 5 of the accompanying drawings is a process flow chart illustratingone form of the method of the invention for recovering copper.

In FIG. 5 the plant 10 which is shown in FIG. 1 and which is describedhereinbefore bears the same reference numeral. The oxygen and carbondioxide sources respectively bear the reference numerals 32 and 34. Thecopper bearing sulphide slurry is labelled with the numeral 50.

The flow sheet in FIG. 5 is an example of the invention applied tocopper bearing sulphide minerals, arsenical sulphides such as enargite,as well as copper bearing sulphide minerals which are refractory tomesophile leaching, such as chalcopyrite.

Copper bearing sulphide concentrate slurry 50 is leached in the plant 10which contains one or more bioleach reactors, using oxygen enriched gasor substantially pure oxygen 32 as the oxidant. The oxygen concentrationin the reactor is controlled in a manner which has been describedhereinbefore depending on the type of microorganism used. The plant 10produces a bioleach residue slurry 100 which contains solubilisedcopper, and iron predominantly in the ferric state.

If the copper bearing sulphide concentrate 50 contains arsenical coppersulphide minerals such as enargite then the bioleach residue 100 willcontain solubilised arsenic. In this instance the residue 100 issubjected to a liquid-solid separation step 102 producing solids 104 fordisposal and solution 106, which is fed to a pH adjustment step 108 inwhich the pH of the solution is adjusted by the addition of limestone110, resulting in partial iron removal by precipitation. Arsenic whichis present in the slurry is also precipitated.

Carbon dioxide 114 produced in the step 108 may be fed to the slurry inthe plant 10 by being blended with oxygen from the source 32 or withcarbon dioxide from the source 34, or by being injected directly intothe slurry in the plant.

Slurry 116 produced by the step 108 is returned to the main flow line.

If the ferric iron-to-copper ratio in solution is unfavourable forsolvent extraction it may also be desirable to carry out the step 108directly on the bioleach residue 100.

The residue 100, or the slurry 116, as the case may be, is thensubjected to a liquid/solid separation step 118 producing solids 120 fordisposal, and a solution 122. The solution in turn is fed to a solventextraction step 124. Strip liquor 126 from the solvent extraction stepis obtained by stripping the loaded solvent with spent electrolyte 128from a copper electrowinning step 130 which produces copper metalcathodes 132. Oxygen gas 134 generated at the anode in theelectrowinning process is fed to the source 32 to supplement the supplyof oxygen to the plant.

Raffinate 136 from the solvent extraction step 124 is neutralised (138)by the addition of limestone 140 and the resulting slurry 142 isdisposed of. A portion of the raffinate may optionally be recycled tothe bioleach step 10 or, if appropriate, to an external heap leach 144,to satisfy acid requirements of these process.

Optionally, if there is insufficient carbonate in the slurry 50, carbondioxide 146 which is generated in the neutralisation step 138 may be fedto the slurry in the plant 10 eg. by being blended with the gas streamfrom the source 32 or by being added to the carbon dioxide source 34.

FIG. 6 illustrates another example of the invention wherein apre-leaching step is applied to a copper bearing sulphide concentrate.Arsenical copper sulphide such as enargite can also be handled in themanner shown in FIG. 6.

The bioleaching plant 10 of FIG. 1 again bears the reference numeral 10in FIG. 6 and the oxygen and carbon dioxide sources respectively bearthe reference numerals 32 and 34.

Copper bearing sulphide concentrate 150 may be pre-leached in one ormore pre-leach reactors 152 using a stream 154 of an acidic solution ofcopper and ferric sulphate which is produced in a manner describedhereinafter.

A slurry 156 produced by the pre-leaching stage 152 is then subjected toa liquid/solid separation step 158 producing residue solids 160 whichare fed to the bioleaching plant 10, and a solution 162.

If the concentrate 150 contains arsenical copper sulphide minerals suchas enargite then the solution 162 and stream 154 each containsolubilised arsenic. In this instance the solution 162 is fed to a pHadjustment step 164 in which the pH of the solution is raised by theaddition of limestone 165 and results in iron removal by precipitation.Arsenic which is present in the slurry is also precipitated.

A slurry 166 emerging from the step 164 is then subjected to aliquid/solid separation step 168 producing solids 170 for disposal and asolution 172 which is returned to the main flow line.

If the solution 162 and stream 154 do not contain arsenic then thesolution 162 is fed to a solvent extraction step 173, which is describedlater.

The residue slurry 160, which contains non-reacted copper and othersulphide minerals, is leached in the plant 10 which contains one or morebioleach reactors using oxygen enriched gas or substantially pure oxygen32, as the oxidant, in the manner which has been described hereinbefore.The oxygen concentration in the reactor is controlled to a suitablevalue, depending on the type of mircroorganism used.

The bioleaching process produces a bioleach residue slurry 174 whichcontains solubilised copper and iron predominantly in the ferric state.

The bioleach residue 174 is subjected to a liquid/solid separation step176 producing solids 178 for disposal and the solution 154 ofsolubilised copper and iron which is used in the pre-leaching stage 152.

The solution 162 is fed to the solvent extraction step 173. Strip liquor190 from the solvent extraction step is obtained by stripping the loadedsolvent with spent electrolyte 192 from a copper electrowinning step 194which produces copper metal cathodes 196. Oxygen gas 198 generated atthe anode during the electrowinning process is directed to the slurry inthe plant 10, for example by being added to the gas stream from theoxygen source 32.

Raffinate 200 produced during the stage 173 is neutralised (202) withlimestone 204 and the resulting slurry 206 is disposed of. A portion ofthe raffinate may optionally be recycled to the bioleach plant 10 or, ifavailable, to an external heap leach 208 to satisfy acid requirements ofthose processes. Carbon dioxide 210 produced in the neutralisation step202 may be directed to the slurry in the plant 10, for example by beingadded to the gas stream from the oxygen source 32 or by being added tothe carbon dioxide from the source 34. Carbon dioxide 212 produced inthe step 164 may be similarly handled.

Particular Example

Bioleach pilot plant test work was completed, using a chalcopyriteconcentrate assaying 32% copper (75% chalcopyrite), on a ˜1.1 m³ pilotplant consisting of 6 reactors configured as 2 primary reactors inparallel followed by 4 secondary reactors in series. The total primaryvolume was 470 l and the total secondary volume was 630 l. All test workwas carried out at a temperature of from 77° C. to 80° C. using a feedslurry containing 10% solids. The microorganisms used were a mixedSulfolobus-like archaea. The oxygen utilisation results obtained in theprimary stage during the test work, using analysis of inlet and outletgas mixtures, are shown in Table 3.

TABLE 3 Primary Reactor Copper Dissolution and Oxygen Uptake Results forThermophile Pilot Test Work Specific Oxygen Cu Cu Dissolution UptakeOxygen Uptake Retention Dissolution Rate (calculated) (measured) Days %kg/m³/h kg/m³/h kg/m³/h 2.8 60.5 0.312 0.668 0.638 2.4 55.9 0.336 0.7180.704

Minimal chalcopyrite leaching, possibly reaching 35% copper dissolution,is found to occur at 40° C. using mesophiles.

What is claimed is:
 1. A method of recovering copper from a copperbearing sulphide mineral slurry which includes the steps of: (a)subjecting the slurry in a reactor to a bioleaching process at atemperature in excess of 40° C.; (b) supplying a feed gas which containsin excess of 21% oxygen by volume, to said slurry; (c) controllingdissolved oxygen concentration in said slurry at a level of from0.2×10⁻³ kg/m³ to 10×10−3 kg/m³ by controlling at least one of thefollowing: an oxygen content of said feed gas; a feed gas supply rate; arate of feed of said slurry to said reactor; and (d) recovering copperfrom a bioleach residue of the bioleaching process.
 2. The methodaccording to claim 1 further including pre-leaching said slurry prior tosaid bioleaching process of step (a).
 3. The method according to claim 2wherein said the pre-leaching is effected using an acidic solution ofcopper and ferric sulphate.
 4. The method according to claim 1 furtherincluding removing ferric arsenate from said bioleach residue beforestep (d).
 5. The method according to claim 4 further including removingferric arsenate by precipitation.
 6. The method according to claim 1further including subjecting said bioleach residue to a neutralisationstep which produces carbon dioxide which is fed to said feed gas of step(b) or directly to said slurry.
 7. The method according to claim 1furthering including recovering copper in step (d) using a solventextraction and electrowinning process.
 8. The method according to claim7 further including feeding oxygen generated during the electrowinningprocess to said feed gas of step (b), or directly to said slurry.
 9. Themethod according to claim 7 further including supplying raffinate,produced during the solvent extraction, to at least one of thefollowing: said bioleaching process of step (a), and an external heapleach process.
 10. The method according to claim 7 further includingfeeding oxygen generated during said electrowinning process to said feedgas of step (b) or directly to said slurry.
 11. The method according toclaim 1 wherein said slurry contains at least one of the following:arsenical copper sulphides, and copper bearing sulphide mineralsrefractory to mesophile leaching.
 12. The method according to claim 11wherein said slurry contain chalcopyrite concentrates.
 13. The methodaccording to claim 1 wherein said feed gas contains in excess of 85%oxygen by volume.
 14. The method according to claim 1 further includingcontrolling a carbon content of said slurry.
 15. The method according toclaim 1 further including controlling a carbon dioxide content of saidfeed gas in a range of from 0.5% to 5.0% by volume.
 16. The methodaccording to claim 1 wherein said bioleaching process is carried out ata temperature in a range of from 40° C. to 100° C.
 17. The methodaccording to claim 16 wherein said temperature is in a range of from 60°C. to 85° C.
 18. The method according to claim 1 further includingbioleaching said slurry at a temperature of up to 45° C. using mesophilemicroorganisms.
 19. The method according to claim 18 wherein saidmicroorganisms are selected from the genus groups comprisingAcidithiobacillus; Thiobacillus; Leptosprillum; Ferromicrobium; andAcidiphilium.
 20. The method according to claim 19 wherein saidmicroorganisms are selected from the group comprising Acidithiobacilluscaldus; Acidithiobacillus thiooxidans; Acidithiobacillus ferrooxidans;Acidithiobacillus acidophilus; Thiobacillus prosperus; Leptospirillumferrooxidans; Ferromicrobium acidophilus; and Acidiphilium cryptum. 21.The method according to claim 1 further including bioleaching saidslurry at a temperature of from 45° C. to 60° C. using moderatethermophile microorganisms.
 22. The method according to claim 21 whereinsaid microorganisms are selected from the genus groups comprisingAcidithiobacillus; Acidimicrobium; Sulfobacillus; Ferroplasma; andAlicyclobacillus.
 23. The method according to claim 22 wherein saidmicroorganisms are selected from the group comprising Acidithiobacilluscaldus; Acidimicrobium ferrooxidans; Sulfobacillus acidophilus;Sulfobacillus disulfidooxidans, Sulfobacillus thermosulfidooxidans;Ferroplasma acidarmanus; Thermoplasma acidophilum; and Alicyclobacillusacidocaldrius.
 24. The method according to claim 17 further includingbioleaching said slurry at a temperature of from 60° C. to 85° C. usingthermophilic microorganisms.
 25. The method according to claim 24wherein said microorganisms are selected from the genus groupscomprising Acidothermus, Sulfolobus; Metallosphaera; Acidianus;Ferroplasma; Thermoplasma; and Picrophilus.
 26. The method according toclaim 25 wherein said microorganisms are selected from the groupcomprising Sulfolobus metallicus; Sulfolobus acidocaldarius; Sulfolobusthermosulfidooxidans; Acidianus infernus; Metallosphaera sedula;Ferroplasma acidarmanus; Thermoplasma acidophilum; Thermoplasmavolcanium; and Picrophilus oshimae.
 27. A plant for recovering copperfrom a copper bearing sulphide mineral slurry which includes a reactorvessel, a source which feeds a copper bearing sulphide mineral slurry tosaid vessel wherein a bioleaching process is carried out at atemperature in excess of 40° C., an oxygen source which supplies oxygenin a form of oxygen enriched gas or substantially pure oxygen to saidslurry, a device which measures a dissolved oxygen concentration in saidslurry in said vessel, a control mechanism whereby, in response to saidmeasured dissolved oxygen concentration, the supply of oxygen from saidoxygen source to said slurry is controlled to achieve a dissolved oxygenconcentration in said slurry of from 0.2×10⁻³ kg/m³ to 10×10⁻³ kg/m³,and a recovery system which recovers copper from a bioleach residue fromsaid reactor vessel.
 28. The plant according to claim 27 furtherincluding a pre-leaching stage for leaching said copper bearing sulphidemineral slurry before said slurry is fed to said reactor vessel.